1. Field of Invention
This invention relates to an electric discharge machining electrode suitable as an electrode used for an electric discharge machine such as an engraving electric discharge machine, a wire electric discharge machine, a micro electric discharge machine, a generating electric discharge machine, and the like. These machines are adapted to subject an article to an electric discharge machining process by generating electric discharge pulses between the article and an electrode disposed at a distance from the article. This invention also relates to an electric discharge machining apparatus, and more particularly to an electric discharge machining electrode capable of improving the quality of a finished surface of an article and/or increasing the machining speed. Optimization of the machining speed may be performed by improving the electric discharge generation rate xcex7, even in a fine machining process, and even when the conditions including a time width and a voltage of the electric discharge pulses applied to the electric discharge machining electrode are the same. Optimization of the machining speed may also be performed by rendering it possible to maintain the electric discharge generation rate xcex7, even when a time width and an applied voltage of each pulse are reduced. This invention also relates to an electric discharge machine. The present invention is preferably used for a micro electric discharge machining process.
2. Description of Related Art
In an electric discharge machining process, when a time width of pulses (which will hereinafter be referred to simply as a pulse width) of a voltage (which will hereinafter be referred to as a gap voltage) applied to a position between an article and an electric discharge machining electrode is large, the quality of the finished surface of the article lowers. Therefore, the pulse width is set to a small value in accordance with the level of required quality of the finished product.
However, when the pulse width is set to a small value, failure in the generation of the electric discharge may occur frequently, even when a voltage pulse is applied between the article and the electrode. Especially, when the area of the article that is directly opposite to the electrode is small, and when the gap voltage is low, failure occurs very noticeably.
The causes of such a failure in the generation of an electric discharge reside in the fact that a long time is needed to electrolytically dissociate any interelectrode substance existing between an article and an electric discharge electrode, i.e., for instance, processing liquid, such as pure water or oil.
The electric discharge generation rate xcex7 will be defined as follows.
Electric Discharge Generation Rate xcex7={(Number of Generations of Electric Discharges)/(Number of Applied Pulses)}xc3x97100 [%]
In the above equation, the  less than Number of Applied Pulses greater than  is the number of applied pulses of a gap voltage, and the  less than Number of Generations of Electric Discharges greater than  is the number of electric discharge pulses generated when the voltage pulses are applied between an article and an electrode. Accordingly, in a case where electric discharge pulses occur at all times when the pulses of a gap voltage are applied between an article and an electrode, the electric discharge generation rate xcex7 in the above equation becomes 100%.
For instance, when a gap voltage and a pulse width of the applied voltage are 100V and 10 nano-seconds respectively, an electric discharge generation rate xcex7 is only about 1% at the highest. However, when the pulse width is 10 micro-seconds with the gap voltage at the same level, the electric discharge generation rate xcex7 becomes very close to 100%. It is true that, when the pulse width is 10 nano-seconds, a treated surface close to a specular surface may be obtained but, when the pulse width is 10 micro-seconds, the treated surface may become extremely rough.
It has been ascertained that, even when the pulse width is set to a small value, the electric discharge generation rate xcex7 can be improved when the gap voltage is increased. However, when the gap voltage is increased, the quality of the treated surface lowers.
When an article has a larger size, the area opposite the electric discharge machining electrode becomes wide, so the electric discharge generation rate xcex7 increases as compared with a case where an article has a small area opposite the electric discharge machining electrode. It is generally said that, when the area of an article which is opposite such an electrode is up to 0.2 mmxc3x9750 mm, the electric discharge generation rate xcex7 is improved in exponential proportion to the area. Such relation between the electric discharge generation rate xcex7 and the area of the article opposite the electric discharge machining electrode is called the xe2x80x9carea effect.xe2x80x9d
However, since the fine electric discharge machining of an article of a small area of 5 xcexcmxc3x975 xcexcm to 25 xcexcmxc3x97100 xcexcm is also much in demand, it is necessary that the electric discharge generation rate xcex7 for an article with a small area does not decrease either.
In view of the above, when a finely machined finished product of high quality having a sub-specular surface is demanded, it is unavoidable that the electric discharge generation rate xcex7 decreases. This necessarily causes the machining rate to decrease.
The present invention has been made in view of the above-described problems and to address the problems encountered in techniques of this kind, and aims at providing an electric discharge machining electrode capable of improving the quality of a finished surface of an article and/or increasing the machining speed. The present invention also improves the electric discharge generation rate xcex7, even in a fine machining process, and even when the conditions including a time width and a voltage of the electric discharge pulses applied to the electric discharge machining electrode are the same, and by rendering it possible to maintain the electric discharge generation rate xcex7 substantially constant even when a time width and an applied voltage of each pulse are reduced. The present invention is also directed to an electric discharge machining apparatus.
The present invention solves these problems by providing an electric discharge machining electrode, used as an electrode for the machining of a surface of an article carried out by generating electric discharge pulses between the article and the electrode, which is located at a distance from the article, wherein a radioactive metal is contained at least in or near the portion of the electrode in which the electric discharge occurs.
In this electric discharge machining electrode, the above-mentioned problems are addressed by forming the electrode out of a superhard material obtained by sintering a mixture containing at least a fine powder of tungsten and cobalt, turning the superhard material into a radioactive metal, as mentioned above, by irradiating the superhard material with neutrons and thereby substituting radioactive cobalt atoms for the above-mentioned cobalt atoms, and using the radioactive metal as a negative pole. The electric discharge machining electrode according to the present invention is different from a conventional electric discharge machining electrode with a superhard material in the fact that the electrode in this invention is radioactive. Also, the mechanical characteristics, such as rigidity, as well as the electrical characteristics of the electrode according to the present invention can be set identical to the mechanical and electrical characteristics of a conventional electrode, which exhibits proven results in an electric discharge machining process. Therefore, the reliability of the electrode according to the present invention during its use as an electric discharge machining of an article can be ensured.
Also, the above-mentioned problems may be addressed by providing an electric discharge machining apparatus, provided with an electric discharge machining electrode as described above.
In the electric discharge machining electrode according to the present invention, a radioactive metal is contained at least in or near the portion of the electrode in which electric discharge pulses occur. The radiant rays emitted by the radioactive metal contained in the electrode exert influence upon the substances existing between an article and the electric discharge machining electrode, which are usually processing liquids such as pure water or oil. The radiant rays emitted by the radioactive metal promote the electrolytic dissociation of these substances. The electrode according to various embodiments of the present invention is formed so as to induce the electric discharge used for the machining of an article, with the radiant rays utilized as a trigger. The radiant rays in use may be beta rays, formed of electrons.
Accordingly, the electric discharge occurs easily between the article and the electric discharge machining electrode. As mentioned above, when the area of the article opposite the electrode is small, the required quality of the treated article is high. Therefore, even when it is necessary to reduce the pulse width or gap voltage to a low level, the electric discharge generation rate xcex7 can be improved.
According to various exemplary embodiments of the present invention, the improvement of the quality of a finished surface of an article can be done by improving the electric discharge generation rate xcex7 even in a fine machining process and even when the conditions including a time width and a voltage of the electric discharge pulses applied to the electrode for the electric discharge machining process are the same, and thereby improving the machining rate. The improvement of the quality of a finished surface can also be improved by rendering it possible to maintain substantially constant the electric discharge generation rate xcex7, even when the time width of the pulses and applied voltage are reduced.
The portion of the electric discharge machining electrode in which a radioactive metal is contained may be any portion, as long as the portion can induce the electric discharge by the radiant rays emitted by the radioactive metal as mentioned above. The present invention does not specifically limit the portion of the electrode to the above-mentioned portion. The size of the portion also depends upon the intensity of the radioactivity of the radioactive metal. The portion of the electrode in which the radioactive metal is contained need not be the entire electrode. Generally, the portion of the electrode in which a radioactive metal is to be contained is preferably at least a portion in which electric discharge pulses occur.